Variable compression ratio in an internal combustion engine is a useful feature which can contribute to better fuel economy. This can be accomplished by varying the compression ratio as a function of engine speed and load so as to maximize the efficiency of the thermodynamic cycle without inducing engine knock. A straightforward way to vary the compression ratio is by varying the TDC (top dead center) clearance volume. This invention is directed to a mechanism for varying the clearance volume by varying the crank radius by use of an eccentric moved in response to variable control fluid pressure and utilizing the engine torsional forces transmitted from the piston to the crankshaft.